Seasonal Changes of Shoot Nitrogen Concentrations and ¹⁵N/¹⁴N Ratios in Common Reed in a Constructed Wetland

Abstract

Nitrogen (N) concentrations and stable N isotope abundances (δ¹⁵N) of common reed (Phragmites australis) planted in a constructed wetland were measured periodically between July 2001 and May 2002 to examine their seasonal variations in relation to N uptake and N translocation within common reed. Nitrogen concentrations in P. australis shoots were higher in the growing stage (7.5 to 24.8 g N kg⁻¹) than in the senescence stage (4.2 to 6.8 g N kg⁻¹), indicating N translocation from shoots to rhizomes. Meanwhile, the corresponding δ¹⁵N values were higher in the senescence stage (+12.2 to +22.4‰) than in the growing stage (+5.1 to +11.3‰). Coupled with the negative correlation (R²=0.24, P<0.05, n=18) between N concentrations and δ¹⁵N values of shoots in the senescence stage, our results suggested that shoot N became enriched in ¹⁵N due to N isotopic fractionation (with an isotopic fractionation factor, α_s/p, of 1.012) during N translocation to rhizomes. However, the positive correlation between N concentrations and δ¹⁵N values in the growing stage (R²=0.19, P<0.001, n=54) suggested that P. australis relies on N re‐translocated from rhizome in the early growing stage and on mineral N in the sediment during the active growing stage. Therefore, seasonal δ¹⁵N variations provide N‐isotopic evidence of N translocation within and N uptake from external N sources by common reed.

Keywords:

• Nitrogen isotope fractionation
• natural ¹⁵N abundance
• nitrogen translocation
• Phragmites australis
• wetland

Acknowledgments

This work was supported by a post‐doctoral fellowship awarded to the senior author by the Korea Science & Engineering Foundation (KOSEF). We gratefully acknowledge the assistance of Mr. S. I. Yun and Mr. Y. D. Choi during the field work and sample preparation. We thank Dr. S. M. Lee for helping with the ¹⁵N analysis.